In general, the term organic light emitting phenomenon refers to a phenomenon in which electric energy is converted to light energy by means of an organic material. The organic light emitting device (OLED) using the organic light emitting phenomenon has a structure usually comprising an anode, a cathode, and an organic material layer interposed therebetween. Herein, the organic material layer may be mostly formed in a multilayer structure comprising the layers consisting of different materials, for example, a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer, and an electron injecting layer, in order to improve efficiency and stability of the organic light emitting device. In the organic light emitting device having such a structure, when a voltage is applied between two electrodes, holes from the anode and electrons from a cathode are injected into the organic material layer, the holes and the electrons injected are combined together to form excitons. Further, when the excitons drop to a ground state, light is emitted. Such the organic light emitting device is known to have characteristics such as self-luminescence, high luminance, high efficiency, low driving voltage, wide viewing angle, high contrast and high-speed response.
Various types of an organic light emitting device are disclosed in the related art, they can be used for different uses. The organic light emitting devices are commonly categorized as a top emission type organic light emitting device, a bottom emission type organic light emitting device and a dual emission type organic light emitting device.
In the case of using a bottom emission type organic light emitting device in the active matrix display, a thin film transistor (TFT) is disposed in front of light emitting source to reduce the aperture ratio of the display. The problem comes to be more serious in the case of producing a more delicate display which needs a large number of TFT. A bottom emission type organic light emitting device, in general, has an aperture ratio of less than 40%. For example, a 14 inch WXGA TFT display has an aperture ratio, which is estimated, of less than 20%. Such a low aperture ratio deteriorates the power consumption for operating OLED and its lifetime.
The above problems can be solved by using a top emission type organic light emitting device. In the top emission type organic light emitting device, the electrode not in contact with a lower substrate, that is, a top electrode, is substantially transparent in the range of visible ray. The transparent material for forming a top electrode in the top emission type organic light emitting device, for example, is a conductive oxide such as IZO (indium zinc oxide) or ITO (indium tin oxide). On the other hand, the electrode in contact with a substrate is usually formed of a metal. A dual emission type organic light emitting device also has a transparent top electrode in the same as a top emission type organic light emitting device.
FIGS. 1 and 2 each illustrate the general structure of the lower portion in an organic light emitting device having a reverse structure and a forward structure. As shown in FIGS. 1 and 2, on manufacturing a top emission type organic light emitting device, the deposition of a metal electrode on a substrate makes undesirably a native oxide layer on the metal electrode. In particular, in the production process of an organic light emitting device, a native oxide layer is formed on a metal electrode by exposing to the external moisture and oxygen during patterning the metal electrode using technology such as a photolithography and an etching process.
The native oxide layer disrupts the properties of the metal electrode, that is an electron injecting property in FIG. 1 and a hole injecting property in FIG. 2, thereby deteriorating the efficiency and luminance of the organic light emitting device.
One of the methods to prevent the native oxide layer from forming on the metal electrode is a method that an organic material layer can be formed in situ on the deposited metal electrode. In the method, the metal electrode is not exposed to air. Therefore, the oxide layer is not formed on the metal electrode. However, it must be performed under vacuum, thereby requiring a high cost and a complicated process. Further, the raw material suppliers often supply a substrate, on which a metal electrode is deposited, in exposing to air, before deposition of an organic material.
Accordingly, despite the presence of a native oxide layer being formed on the metal electrode, the development for a top emission type organic light emitting device, which has the improved electron injecting or hole injecting property, and a method for producing the same has been needed.